1. Field of Invention
The present invention relates to a power converter, and more particularly, to a switching control circuit of the power converter.
2. Description of Related Art
Power converters are used to convert an unregulated power source to a regulated voltage or current source. The power converter normally includes a transformer or a magnetic device having primary winding and secondary winding to provide the isolation. A switching device connected in the primary winding to control energy transfer from the primary winding to the secondary winding. The power converter operates at a high frequency for allowing a size and weight reduction. However, the switching of the switching device generates the switching losses and electric-magnetic-interference (EMI). FIG. 1 shows a flyback power converter and the waveforms are shown in FIG. 2. The switching device 20 is applied to switch a transformer 10 and control the power delivered from the primary winding to the secondary winding of the transformer 10. Energy is stored into the transformer 10 when the switching device 20 is turned on. As the switching device 20 is switched off, the energy of the transformer 20 is discharged to the output of the power converter through a rectifier 30. In the mean time, a reflected voltage VR is generated in the primary winding of the transformer 10 in accordance with the output voltage VO and the turn-ratio of the transformer 10. Therefore, the voltage VD across to the switching device 20 is equal to the input voltage VIN plus the reflected voltage VR once the switching device 20 is turned off. The energy from the voltage VD is stored into the parasitic capacitor 25 of the switching device 20. After a discharge period TDS, the energy of the transformer 10 is fully discharged, the energy stored in the parasitic capacitor 25 flows back to the input voltage VIN through the primary winding of the transformer 10. The parasitic capacitor 25 (capacitance Cj) and the primary winding inductor (inductance LP) of the transformer 10 develop a resonant tank, wherein its resonant frequency fR can be shown as equation (1),
                              f          R                =                  1                      2            ⁢            π            ⁢                                                            L                  p                                ×                                  C                  j                                                                                        (        1        )            During the resonant period, the energy of the parasitic capacitor 25 is delivered to the primary inductor of the transformer 10 back and forth. From the parasitic capacitor 25 discharged to a valley voltage occurred on the voltage VD, there is a delay time Tq. The delay time Tq is the period of the quasi-resonant and it can be expressed as equation (2),
                              T          q                =                  1                      4            ×                          f              R                                                          (        2        )            If the switching device 20 can be turned on during the valley voltage across the switching device 20, which can achieve the soft switching to minimize the switching loss and EMI.
The object of the present invention is to develop a switching control circuit that is able to soft switch the transformer and improve the efficiency of the power converter.